1. Field of the Invention
This invention relates to photolithography and more particularly to proximity correction in the presence of subresolution assist features used in photolithography.
2. Description of Related Art
A very large scale integrated (VLSI) complementary metal oxide semiconductor (CMOS) chip is manufactured on a silicon wafer by a sequence of material additions (i.e., low pressure chemical vapor depositions, sputtering operations, etc.), material removals (i.e., wet etches, reactive ion etches, etc.), and material modifications (i.e., oxidations, ion implants, etc.). These physical and chemical operations interact with the entire wafer. For example, if a wafer is placed into an acid bath, the entire surface of the wafer will be etched away. In order to build very small electrically active devices on the wafer, the impact of these operations has to be confined to small, well defined regions.
Lithography in the context of VLSI manufacturing of CMOS devices is the process of patterning openings in photosensitive polymers (sometimes referred to as photoresists or resists) which define small areas in which the silicon base material is modified by a specific operation in a sequence of processing steps. The process of manufacturing of CMOS chips involves the repeated patterning of photoresist, followed by an etch, implant, deposition, or other operation, and ending with the removal of the expended photoresist to make way for the new resist to be applied for another iteration of this process sequence.
The basic lithography system consists of a light source, a stencil or photo mask containing the pattern to be transferred to the wafer, a collection of lenses, and a means for aligning existing patterns on the wafer with patterns on the mask. The aligning may take place in an aligning step or steps and may be carried out with an aligning apparatus. Since a wafer containing from 50 to 100 chips is patterned in steps of 1 to 4 chips at a time, these lithography tools are commonly referred to as steppers. The resolution, R, of an optical projection system such as a lithography stepper is limited by parameters described in Raleigh's equation:R=kλ/NA, where λ represents the wavelength of the light source used in the projection system and NA represents the numerical aperture of the projection optics used. “k” represents a factor describing how well a combined lithography system can utilize the theoretical resolution limit in practice and can range from about 0.8 down to about 0.5 for standard exposure systems. The highest resolution in optical lithography is currently achieved with deep ultra violet (DUV) steppers operating at 248 nm. Wavelengths of 356 nm are also in widespread use and 193 nm wavelength lithography is becoming commonplace.
Conventional photo masks consist of chromium patterns on a quartz plate, allowing light to pass wherever the chromium has been removed from the mask. Light of a specific wavelength is projected through the mask onto the photoresist coated wafer, exposing the resist wherever hole patterns are placed on the mask. Exposing the resist to light of the appropriate wavelength causes modifications in the molecular structure of the resist polymers which, in common applications, allow a developer to dissolve and remove the resist in the exposed areas. Such resist materials are known as positive resists. (Negative resist systems allow only unexposed resist to be developed away.) The photo masks, when illuminated, can be pictured as an array of individual, infinitely small light sources which can be either turned on (points in clear areas) or turned off (points covered by chrome). If the amplitude of the electric field vector which describes the light radiated by these individual light sources is mapped across a cross section of the mask, a step function will be plotted reflecting the two possible states that each point on the mask can be found (light on, light off).
These conventional photo masks are commonly referred to as Chrome-on-Glass (COG) binary masks, due to the binary nature of the image amplitude. The perfectly square step function of the light amplitude exists only in the theoretical limit of the exact mask plane. At any given distance away from the mask, such as in the wafer plane, diffraction effects will cause images to exhibit a finite image slope. At small dimensions, that is, when the size and spacing of the images to be printed are small relative to the λ/NA, electric field vectors of adjacent images will interact and add constructively. The resulting light intensity curve between the image features is not completely dark, but exhibits significant amounts of light intensity created by the interaction of adjacent features. The resolution of an exposure system is limited by the contrast of the projected image, that is, the intensity difference between adjacent light and dark image features. An increase in the light intensity in nominally dark regions will eventually cause adjacent features to print as one combined structure rather than discrete images.
The quality with which small images can be replicated in lithography depends largely on the available process latitude; that is, that amount of allowable dose and focus variation that still results in correct image size.
Sub-Resolution Assist Features (SRAF), also known as scattering bars, intensity leveling bars and assist bars, referred to hereinafter as SRAF elements have been demonstrated to yield significant improvement in the lithographic process window when used in conjunction with Off-Axis Illumination (OAI) J. Bruce, M. Cross, L. Liebmann, S. Mansfield, and A. McGuire, entitled “Assist Features—Challenges and Opportunities”, Proceedings of the Microlithography Symposium Interface 2000 Sponsored by Arch Chemicals, Inc. Nov. 5–7, 2000 San Diego, Calif. See also U.S. Pat. No. 5,242,770 of Chen et al. for “Mask for Photolithography” and U.S. Pat. No. 5,821,014 of Chen for “Optical Proximity Correction Method for Intermediate-pitch Features Using Sub-Resolution Scattering Bars on a Mask”.
Methodologies for generating rules for the placement and size of SRAF elements are known and have been described in U.S. Pat. No. 6,421,820 of Mansfield et al. entitled “Semiconductor Device Fabrication Using a Photomask with Assist Features” and in an article by Mansfield et al. entitled “lithographic Comparison of Assist Feature Design Strategies” Proc. of SPIE Vol. 4000, Optical Microlithography XIII (March, 2000) pp. 63–76.
Challenges in fitting the inherently one-dimensional SRAF elements into two-dimensional circuit layouts are described in: Liebmann et al. “Optimizing Style Options for Sub-Resolution Assist Features,” in Proc. SPIE, Vol. 4346, SPIE, (2001). This article describes clean up rules for insuring manufacturability and good image quality and describes the negative effects of locally missing SRAF elements on the print quality of the primary circuit patterns. Also mentioned are challenges in integrating the SRAF design with model-based approaches.
U.S. Pat. No. 6,413,683 Liebmann et al. for “Method for Incorporating Sub Resolution Assist Features in a Photomask Layout” describes style options used to clean up mask designs to insure manufacturability and image quality.
Also, see Liebmann et al. “TCAD Development for Lithography Resolution Enhancement” IBM J. RES. DEV. VOL. 45, No. 5, September 2001 pages 651–665 shows a partial SRAF rules table. In addition, see Liebmann, L. W. “Resolution Enhancement Techniques in Optical Lithography, It's Not Just a Mask Problem”, Proceedings of SPIE—The International Society for Optical Engineering Vol. 4409 (2001) p. 23–32.
None of the above patents or the above articles discusses proximity correction of subresolution assist features used in photolithography.